Method for reducing hydrogen sulfide emissions from wastewater

ABSTRACT

A cost-effective method for reducing the dissolved sulfide content in a wastewater stream and thereby hydrogen sulfide emissions therefrom involving the steps of adding a transition metal salt to the wastewater stream at the upper reaches of a wastewater collection system prior to at least some hydrogen sulfide volatilization followed by addition of an oxidant to the wastewater stream to generate elemental sulfur and a transition metal salt which subsequently participates in additional hydrogen sulfide capturing steps, thereby also improving water quality and wastewater treatment plant operations.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for reducing thedissolved sulfide content within a wastewater stream and thereby thehydrogen sulfide emissions there from and for improving water qualityand treatment plant operations by employing a transition metal salt andan oxidant as additives to the wastewater stream.

[0002] Hydrogen sulfide (H₂S) is a toxic, corrosive gas that isgenerated within the biomass adhered to pipe walls and sediment of asewage system. As wastewater is conveyed through the sewage collectionsystem to the wastewater treatment plant, septic conditions develop thatfoster the growth of hydrogen sulfide-producing bacteria. Hydrogensulfide volatilizes from the wastewater into the vapor space of thesewage system where it creates the problems of nuisance odors,infrastructure corrosion, and worker hazards.

[0003] The use of iron salts alone to control hydrogen sulfide emissionsin wastewater is known in the industry. Iron salts control hydrogensulfide (H₂S) by converting volatile H₂S dissolved in the wastewaterinto nonvolatile iron-complexed sulfide (FeS):

H₂S+FeCl₂→FeS+2 HCl.

[0004] Ferrous sulfide (FeS) is a black precipitate that is stable inthe absence of acid and typically settles out in clarifiers at thewastewater treatment plant, where it enters the solids stream. Thestoichiometric chemical requirement is 1.7 pounds Fe (or 3.7 poundsFeCl₂) per pound H₂S controlled, yielding a cost of approximately $0.50per pound hydrogen sulfide depending on the per-unit chemical cost.Additionally the efficiency of iron salts is not impacted by oxygenuptake rates within the wastewater.

[0005] Despite these advantages, the use of iron salts alone to controlH₂S emissions in wastewater has shortcomings. Iron salts alone loseefficiency when achieving H₂S emissions control for more than about fourhours hydraulic retention time. Hydraulic retention time is defined asthe length of time a component resides within the sewage system. Thusthe efficient use of iron salts alone to control H₂S emissions ofwastewater requires a series of iron salt injection facilities locatedalong the course of the wastewater collection system. At each injectionsite, the spent iron salt (FeS) is augmented with fresh iron salt. Asused herein, the term “iron salt” refers to nearly any iron compound (asdistinguished from elemental iron) and expressly includes iron hydroxide(Fe(OH)₂, Fe(OH)₃, FeCl₃, FeCl₂, FeSO₄, and Fe₂(SO₄)₃) but excepts FeS,which is often referred to herein as “spent iron salt.” The spent ironsalt largely remains inert throughout the treatment and disposalprocesses. When the wastewater reaches the treatment plant, the mass ofspent iron salt settles out in the primary clarifiers. The iron saltdemand increases 2-4 fold to achieve H₂S emissions control for greaterthan about four hours hydraulic retention time, thus increasing theamount of spent iron salt generated. The FeS precipitates andconstitutes a theoretical solids load of about 3 pounds per pound H₂Scontrolled. The FeS precipitate can cause deposition problems within thesewage system, particularly in low-velocity sewage systems andclarifiers/thickeners as it settles out, thus increasing the actual costper pound H₂S controlled by 20% ($0.075) or more.

[0006] Iron salts also degrade the quality of wastewater. The salinityof wastewater is increased by the addition of iron salts, as a minimumof 3 pounds sodium chloride per pound H₂S controlled is generated whenFeCl₂ or FeCl₃ is used as the iron salt. Iron salts also deplete thealkalinity of the wastewater stream by consuming a minimum of 3 poundscalcium carbonate per pound H₂S controlled. Further, iron salt productstypically contain 1-4% mineral acid that further depresses the pH of thewastewater. The reduced alkalinity of the wastewater stream in turnreduces the capturing capacity of iron, thus reducing its ability tocontrol H₂S to low levels. Furthermore, the depressed pH of thewastewater encourages volatilization of untreated H₂S within thewastewater stream. Additionally, iron salts deplete the wastewaterstream of dissolved oxygen by consuming a minimum of 5 pounds dissolvedoxygen per pound H₂S controlled. Thus, while iron salts are useful incontrolling H₂S emissions of wastewater, it is desirable to minimize theamount of iron salt added to the wastewater stream to minimize thedisadvantages associated with the use of iron salts.

[0007] It has been reported that a blend of 1 part ferrous to 2 partsferric iron provides improved control of H₂S emissions from a wastewaterstream when compared to either ferrous or ferric iron alone. Such ablend, however, is expensive and is subject to the same disadvantages ofiron salts previously stated.

[0008] The use of hydrogen peroxide (H₂O₂) alone to control H₂Semissions is also conventional. Like iron salts, H₂O₂ injectionfacilities within the sewage system are typically located in series,separated by 1-2 hours hydraulic retention time. The use of hydrogenperoxide alone controls H₂S emissions in wastewater by two mechanisms:direct oxidation of H₂S to elemental sulfur (I) or prevention of H₂Sformation by supplying dissolved oxygen (II):

H₂S+H₂O₂→S+2H₂O  (I)

2H₂O₂→O₂+2H₂O  (II).

[0009] Direct oxidation theoretically requires 1.0 pound H₂O₂ per poundH₂S controlled at a cost of about $0.50 per pound H₂S and generates 1.0pound solids per pound H₂S controlled, regardless of H₂O₂ dose. Incontrast, prevention of H₂S formation by providing a dissolved oxygensupply theoretically requires 4.0 pounds H₂O₂ per pound H₂S controlledat a cost of $2.00 per pound sulfide. The second mechanism also isadversely affected by environmental factors such as hydraulic retentiontime and oxygen uptake. Thus, the practical H₂O₂ requirement can be 2-4times the theoretical H₂O₂ requirement when retention time increases by2-3 hours. Therefore, previous H₂O₂ applications within the municipalwastewater treatment industry are either targeted at point source H₂Scontrol, such as at the headworks to treatment plants, where H₂O₂ may beapplied to the wastewater stream to maximize its most efficient mode asan oxidant, or added as a preventative within the wastewater collectionsystem, at costs exceeding $2.00 per pound H₂S controlled.

[0010] While the independent use of H₂O₂ to control H₂S emissions bywastewater generates no adverse by-products and advantageouslyoxygenates the wastewater, it presents several shortcomings.Specifically, the oxidation reaction typically requires 15-30 minutes.In addition, control of H₂S emissions at 1-2 hours hydraulic retentiontime or more is expensive, requiring double the injection stationsrequired by FeCl₂ control. Furthermore, the efficiency of H₂O₂ isadversely affected by high oxygen uptake rates. Hence, a mechanism whichprovides greater and more efficient H₂S emissions control within thewastewater treatment system is desirable.

[0011] In response to the need for an improved H₂S emission controlmechanism, one study examined the effect of combining iron salt andsodium hypochlorite in a water treatment process. Specifically, sodiumhypochlorite was added at the treatment plant to a force main dischargeto which iron salt had been added at a pump station located upstream ofthe treatment plant. The results of the study demonstrated that such acombination treatment afforded more complete and consistent H₂Semissions control at reduced cost in comparison to the addition of ironsalt alone to the waterstream. This process, however, has never beenapplied to a sewage collection system. Furthermore, this method of H₂Scontrol has not been combined with a process for enhancing solidsremoval at the treatment plant.

[0012] A process for the conversion of aqueous hydrogen sulfide ingeothermal steam employing hydrogen peroxide and iron compounds istaught by U.S. Pat. No. 4,363,215 to Sharp. In the disclosed process,hydrogen sulfide is reacted with hydrogen peroxide, wherein the ironcompound serves as a catalyst to accelerate the reaction of hydrogenperoxide with hydrogen sulfide. The iron compound catalyst is added inan amount of from 0.5 to 1.0 parts per million expressed as free metaland thus does not complex with the sulfide. U.S. Pat. No. 4,292,293 toJohnson et al. further discloses the addition of polyanionic dispersantsto improve the efficiency of the metallic ion catalyst for the oxidationof sulfide by hydrogen peroxide.

[0013] The addition of a combination of a ferric salt and an anionicpolymer to a water clarifier is a known development in enhancing solidsseparation and thereby improving the cost-performance of wastewatertreatment plants, though such treatment has not yet been widely employedwithin the industry.

[0014] It is the object of the present invention to provide acost-effective means for reducing hydrogen sulfide emissions throughoutthe wastewater collection system as well as the wastewater treatmentplant while improving water quality and treatment plant operations.

BRIEF SUMMARY OF THE INVENTION

[0015] The present invention provides a novel method for integrating theuse of transition metal salts and oxidants, specifically iron salts andhydrogen peroxide, respectively, to achieve reduced dissolved sulfidelevels and thereby H₂S levels within a sewage system. Iron salts areadded to the wastewater stream at the upper reaches of the wastewatercollection system prior to H₂S volatilization to capture H₂S dissolvedin the wastewater. The captured sulfide, as ferrous sulfide, is thendelivered to one or more points downstream of the iron salt additionwhere hydrogen peroxide is added to the wastewater stream. The hydrogenperoxide destroys the ferrous sulfide and restores the sulfide-capturingcapacity of the iron. At the final regeneration point, for example thewastewater treatment plant, the restored iron salt is used to enhancesolids separation and sulfide control in primary clarifiers as well assulfide and struvite control in anaerobic digesters.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Applicants have discovered a novel process for controllinghydrogen sulfide emissions from wastewater and for improving waterquality and wastewater treatment plant operations in a cost-effectivemanner. The method of the present invention involves the use oftransition metal salts to capture sulfides generated within a sewagesystem and to deliver the captured sulfides to an oxidant added at apoint downstream of the transition metal salt addition. The oxidantrestores the capturing capacity of the transition metal by regeneratinga transition metal salt from the spent transition metal salt (transitionmetal sulfide), thus allowing the regenerated transition metal salt toparticipate in the capture of additional hydrogen sulfide molecules. Thetransition metal salt is preferably a ferrous or ferric salt in asolution or any readily water-soluble form. For example, the iron saltmay be ferrous or ferric sulfate, chloride, nitrate, bromide, bromate,or a mixture thereof. The oxidant is preferably hydrogen peroxide.

[0017] The transition metal salt, preferably iron salt, is added to awastewater stream at the upper reaches of a wastewater collection systemprior to hydrogen sulfide volatilization. While hydrogen sulfide willbegin volatilizing almost immediately, the term “prior to hydrogensulfide volatilization” is intended to mean prior to somevolatilization, not necessarily prior to all volatilization. The ironsalt will aid in preventing future H₂S volatilization regardless ofwhether H₂S has volatilized previously. It would be practicallyimpossible to introduce iron salt prior to all H₂S volatilization. Thegreater the amount of hydrogen sulfide already present in the wastewaterstream at the point of iron salt addition, the greater the benefit ofadding ferric salt to control hydrogen sulfide emissions, instead offerrous salt, as ferric salt has an oxidizing capacity, albeit small. Anoxidant, preferably hydrogen peroxide, is then added to the wastewaterstream at one or more points downstream of the iron salt addition, toregenerate iron salt in situ. The hydrogen peroxide oxidizes the ironsulfide formed, to produce ferric hydroxide and/or ferrous hydroxidesalts.

[0018] Multiple regeneration steps using a series of hydrogen peroxideadditions spaced at points separated by approximately 4 hours hydraulicretention time may be used where the water collection system is long. Inaddition, hydrogen peroxide is preferably added to the influentwastewater stream at the wastewater treatment plant, the finalregeneration point. The iron hydroxide produced enhances solidsseparation and sulfide control in primary clarifiers, as well as sulfideand struvite control in anaerobic digesters at the treatment plant. Ananionic polyelectrolyte may be added to the influent of a primaryclarifier at the wastewater treatment plant to further improve solidsseparation.

[0019] The present invention may be represented as the followingcatalytic cycle, where a working inventory of iron is maintained withhydrogen sulfide (H₂S) as the input, elemental sulfur (S_(o)) as theoutput, and hydrogen peroxide (H₂O₂) as the driver:

[0020] The preferred embodiment of the process occurs in three steps:(1) iron complexation with dissolved sulfide; (2) direct H₂O₂ oxidationof the FeS complex to provide elemental sulfur and ferric hydroxide(Fe(OH)₃); and (3) oxidation of additional sulfide by the ferrichydroxide to produce elemental sulfur and FeS. The second and thirdsteps are then repeated as additional hydrogen peroxide is used toregenerate ferric hydroxide from the ferrous sulfide. The preferredprocess may be exemplified in the following overall reaction: Step 1: 2H₂S + 2 FeCl₂→2 FeS + 4 HCl Step 2: 2 FeS + 3 H₂O₂→2 S_(o) + 2 Fe(OH)₃Step 3: 2 Fe(OH)₃ + 3H₂S→S_(o) + 2 FeS + 6 H₂O

[0021] This net reaction stoichiometrically requires 0.67 lbs Fe (or1.45 lbs FeCl₂) and 0.6 lbs H₂O₂ per lb sulfide, to yield a theoreticalcost of about $0.50 per lb sulfide controlled. This is based on 0.67 lbsFe²⁺. Commensurately less iron would be required if introduced as Fe³⁺.In that case, only 0.5 lbs Fe³⁺ and 0.6 lbs H₂O₂ per lb sulfidecontrolled would be needed.

[0022] The method of the present invention achieves a number ofadvantages over conventional methods. In sharp contrast to conventionaltreatment techniques the addition of fresh iron salt downstream of theinitial injection site or even at the treatment plant is not required,as a mixture of ferric and ferrous salts is provided by in situregeneration of spent iron salt by hydrogen peroxide in the wastewatercollection system and upon entry of the treatment plant. Because ironsalt need only be added to the wastewater stream at one point in thecollection process and is regenerated thereafter by hydrogen peroxide,the present invention requires only a fraction of the iron inputrequired by the related art. Thus, the present invention achieves agreater than 40% reduction in solids production, a greater than 60%reduction in acidity contribution, and a greater than 80% reduction indissolved oxygen demand. These benefits result in an overall costsavings by reducing the solids load and the amount of iron saltrequired, thus reducing the solids generated and associated disposalcost.

[0023] Additionally, the iron levels in the wastewater stream augmentedby reaction of FeS with H₂O₂ increases the removal rate of H₂S by morethan 90%, thus significantly improving the degree of H₂S controlafforded as compared to the use of iron salt or H₂O₂ alone. For example,the use of H₂O₂ alone to control sulfide emissions requires H₂O₂addition at a site 20-40 minutes hydraulic retention time upstream ofthe point of H₂S release. In sharp contrast, the present inventionallows the addition of H₂O₂ to be located at or 1-10 minutes prior tothe point of desired H₂S control.

[0024] Regeneration of iron salt with H₂O₂ also results in a mixture offerric and ferrous salts having superior H₂S capturing capacity ascompared to ferric or ferrous salt alone. The ferrous-to-ferric ironratio of this blended product may be adjusted by varying the H₂O₂ dose.

[0025] The present invention represents a novel and significantimprovement for reducing H₂S emissions from wastewater. In addition toproviding practical, long-duration H₂S control to low sulfide levels viaa rapid oxidative reaction, the method of the present invention providessignificant treatment plant benefits. The invention being thusdescribed, it will be obvious that the same may be varied in many ways.Such variations are not to be regarded as a departure from the spiritand scope of the invention, and all such modifications as would beobvious to one skilled in the art are intended to be included within thescope of the following claims.

We claim:
 1. A method for reducing the evolution of hydrogen sulfidevapors within a sanitary sewer system, comprising the steps of: addingan iron salt to a wastewater stream within said sanitary sewer systemupstream of hydrogen sulfide volitilization to produce free iron ionswhich react with said hydrogen sulfide to form iron (II) sulfide; andadding an oxidant to said wastewater stream downstream of said iron saltaddition to regenerate free iron ions from said iron (II) sulfide. 2.The method of claim 1 wherein said oxidant is hydrogen peroxide.
 3. Themethod of claim 1 wherein said iron salt is selected from the groupconsisting of ferrous chloride, ferrous sulfate, ferric chloride, ferricsulfate, and mixtures thereof.
 4. The method of claim 1 wherein saidregenerated free iron ions are ferric ions.
 5. The method of claim 1,further comprising the step of adding an anionic polyelectrolyte to saidwastewater stream at said wastewater treatment plant.
 6. A method ofenhancing solids separation in a primary clarifier comprising: adding aniron salt to a wastewater stream in a wastewater collection systemupstream of hydrogen sulfide volatilization to produce free iron ionswhich react with said hydrogen sulfide to form iron (II) sulfide; addingan oxidant to said wastewater stream downstream of said iron saltaddition to regenerate free iron ions from said iron (II) sulfide, whichfree iron ions react with said hydrogen sulfide to reform iron (II)sulfide; and adding an oxidant to said wastewater stream at the inlet ofa wastewater treatment plant prior to entry of said wastewater to saidprimary clarifier.
 7. A method of treating wastewater at a wastewatertreatment plant comprising: adding an iron salt to a wastewater streamin a wastewater collection system upstream of hydrogen sulfidevolatilization to produce free iron ions which react with said hydrogensulfide to form iron (II) sulfide; adding an oxidant to said wastewaterstream downstream of said iron salt addition to regenerate free ironions from said iron (II) sulfide, which free iron ions react with saidhydrogen sulfide to reform iron (II) sulfide; and adding an oxidant tosaid wastewater stream at the inlet of a wastewater treatment plant toregenerate free iron ions from said reformed iron (II) sulfide.
 8. Amethod of enhancing sulfide control in an anaerobic digester comprising:adding an iron salt to a wastewater stream in a wastewater collectionsystem upstream of hydrogen sulfide volatilization to produce free ironions which react with said hydrogen sulfide to form iron (II) sulfide;adding an oxidant to said wastewater stream downstream of said iron saltaddition to regenerate free iron ions from said iron (II) sulfide, whichfree iron ions react with said hydrogen sulfide to reform iron (II)sulfide; and adding an oxidant to said wastewater stream in a wastewatertreatment plant prior to entry of said wastewater to said anaerobicdigester.
 9. A method for reducing the evolution of hydrogen sulfidevapors within a sanitary sewer system, comprising the steps of: addingan iron salt to a wastewater stream within said sanitary sewer systemupstream of hydrogen sulfide volitilization to produce free iron ionswhich react with said hydrogen sulfide to form iron (II) sulfide; makinga first oxidant addition to said wastewater stream downstream of saidiron salt addition to regenerate free iron ions from said iron (II)sulfide; adding an oxidant to said wastewater stream downstream of saidfirst oxidant addition by at least about 4 hours hydraulic retentiontime, and upstream of a wastewater treatment plant.
 10. The method ofclaim 1 wherein said iron salt is added to said wastewater stream in anamount of at least 0.50 pounds Fe per pound sulfide controlledcalculated as pounds H₂S.
 11. The method of claim 2, wherein saidhydrogen peroxide is added to said wastewater stream in an amount of atleast 1.0 lbs H₂O₂ per pound sulfide controlled calculated as poundsH₂S.
 12. The method of claim 10 wherein said iron salt is added to saidwastewater stream in an amount of at least 0.67 pounds Fe per poundsulfide controlled calculated as pounds H₂S.
 13. The method of claim 1wherein said iron salt is added to said wastewater stream in astoichiometric amount calculated based on the amount of sulfidecontrolled.